Biodiversity in the Western Ghats: An Information Kit (IIRR, 1994, 224 p.)

7. Invertebrates

7.1 Butterflies

7.2 Honeybees to conserve biodiversity

7.3 Mulberry silkworms

7.4 Spiders

7.5 Conserving natural enemies of mosquitoes

7.6 Vermicomposting

7.5 Conserving natural enemies of mosquitoes

Mosquitoes are some of nature's most unloved creatures. Their
bites itch, and they transmit dreaded diseases like malaria, filaria, dengue and
Japanese encephalitis. Mosquitoes have enormous reproductive potential: if left
unchecked, a female mosquito could produce 8 billion other females in only two
months.

Fortunately, nature has provided a variety of biological control
agents-pathogens, parasites and predators-that affect every stage of the
mosquito's life cycle. They vary from micro-organisms to mammals such as bats,
and even pitcher plants. Besides these natural enemies, abiotic factors such as
temperature, humidity and rainfall also limit mosquito populations. The net
result is to inflict considerable mortality on each generation of mosquitoes.

Threats to big-control agents

In the quest to eliminate insect pests of crops and insects that
carry diseases, humans have resorted to the indiscriminate use of insecticides
to support both agriculture and health.

Initial successes were tremendous. But through repeated use, the
target insects (including mosquitoes) have gradually acquired physiological
resistance to these insecticides. Many insecticides have a very broad-spectrum
action, so they kill the natural enemies as well as the pests. And some
pesticides enter the food chain, accumulate in the bodies of certain organisms,
and disrupt the food web. These effects have caused incalculable harm to the
natural enemies of mosquitoes and other insect pests.

Conserving, propagating and using biological control agents have
gained tremendous importance. Chemical insecticides should be used only when
absolutely essential, and then with extreme caution.

Promoting natural enemies

Mosquitoes have many natural enemies, but of this long list,
only a few can be used on a large scale. These include toxins of bacteria
(Bacillus sphaericus and Bacillus thuringiensis israeliensis) and certain types
of fish.

Mosquito life cycle

Mosquitoes galore

A female mosquito lays on an average of 150 eggs every alternate
day. Imagine a female lays eggs on 4 occasions in her life span (i.e., a total
of 600 eggs) and half of the eggs hatch into females and the other half into
males. After the 4th generation (i.e., in about two months), each female would
theoretically produce 300 × 300 × 300 × 300 females, or about 8
billion females!

Several indigenous species of fish have been identified and
tested against mosquito larvae in Goal A small fish, Aplocheilus block) or
"kankatre", is very effective at controlling mosquitoes and malaria. This fish
is found throughout Goa in estuaries, fishponds, backwaters, lakes and streams.
It is a much better predator of mosquito larvae than exotic species of fish such
as Gambusia affinis (mosquito fish) and Lebistes reticulates (guppy).
Aplocheilus block) is easily identified by the white spot between the eyes. Four
to five fish per square metre surface area of water effectively control
mosquitoes.

Another very agile, hardy and good larvivorous fish is Rasbora
daniconius, or "dadiyo". These fish occur naturally in groups in ponds and
streams. They are easily identified by a prominent dark line running from the
operculum to the tail fin. People in Goa keep this fish in wells to keep the
water clear of debris and the well walls free of algae and ferns. That they also
predate upon mosquito larvae is not so well known. Two or three fishes per
square metre of surface area can control mosquitoes in ponds, wells and tanks.

Aplocheilus blocki

Rasbora daniconius

Puntius ticto

Aplocheilus lineatus

Number of mosquito larvae consumed per day

Fish species

Max size (cm)

Larvae eaten per day, per gram fish weight

Aplocheilus block)

4

201

Lebistes reticulates

5

112

Gambusia affinis

8

138

Aplocheilus lineatus

10

78

Puntius ticto

10

126

Heteropneustes sp.

15

88

Oreochromis mossambicus

18

128

Rasbora daniconius

20

86

The bigger the fish, the more mosquito larvae it eats. But it is
better to keep many small fish than a few large ones. The table above shows that
in relation to their size, smaller fish eat more larvae than do bigger fish.

Frogs

Frogs are well-known big-control agents of insects. Adult frogs
prey upon flying insects by shooting their sticky tongues out at the prey. The
tadpoles devour aquatic insects, including mosquito larvae. Unfortunately, the
number of frogs is dwindling because of insecticide use. Frogs are also hunted
for their hind legs, which are considered a delicacy. Frogs can control
mosquitoes in ponds. rice fields, lakes and water tanks. They are effective
against mosquito vectors of Japanese encephalitis, particularly during the
monsoon.

Frogs

Turtles

Many villagers in Goa keep freshwater turtles in their wells.
The people believe they bring good luck and help keep the well water clean.
Scientists in the Malaria Research Centre in Goa have found that turtles also
control mosquitoes. In laboratory studies, fist-sized juvenile turtles can
consume about 5000 mosquito immatures per day. Unfortunately, turtles are hunted
for their flesh and carapace; their numbers have been dwindling very rapidly.

Turtles

Lizards

Lizards are often hated in the household as they are thought to
be scary. A careful watch would show that they prey on various insects,
including adult mosquitoes.

Lizards

Spiders

Spiders spin webs on walls, in corners and in dark, secluded
places to trap their prey. Adult mosquitoes can often be seen trapped in the
webs.

Spiders

Need to conserve natural enemies of mosquitoes

Research has shown the usefulness of larvivorous fishes and
bacterial toxins in controlling mosquitoes and the diseases they spread. But we
need to conserve and use natural control agents of mosquitoes at the household
level. These friendly creatures face a constant dual threat from chemical
insecticides and humans. It is common in Goa to see people hunting frogs for
their hind legs immediately after the first pre-monsoon showers. The hunters
fail to realise that killing frogs during this breeding season prevents them
from producing offspring. Public awareness needs to be created to stop the
slaughter of these friendly animals. Similarly, turtles also need to be
protected. People should also use insecticides in a very selective and judicious
manner to avoid harming big-control agents.